1. Field of the Invention
The present invention relates to a temperature measurement circuit and, more particularly, to a temperature measurement circuit capable of performing a calibration function through shifting a conversion reference level of an analog-to-digital conversion circuit.
2. Description of the Related Art
Because the potential difference across the semiconductor pn junction of diodes or transistors is related to the current flowing through the junction itself and further depends on the temperature of the junction, this kind of semiconductor pn junction is widely employed in the integrated circuits to perform the task of temperature measurement. FIG. 1 is a schematic diagram showing a circuit configuration of a conventional temperature measurement circuit 10. Typically, the temperature measurement circuit 10 is installed to monitor a temperature of an external system 20. The external system 20 may, for example, refer to a computer, an electronic device, or a certain circuitry region, which has a thermal sensor 21 built inside to provide a semiconductor pn junction for trying to detect the temperature of the external system 20. As shown in the figure, the thermal sensor 21 may be implemented by a pnp bipolar transistor that provides the semiconductor pn junction between the base and emitter electrodes.
In the temperature measurement circuit 10, two switches S1 and S2 of a current source circuit 11 are turned ON and OFF by a control circuit 12 to therefore allow different currents I1 and I2 to be applied to the thermal sensor 21, respectively. Assumed that the current I1 is applied to the thermal sensor 21 to cause a potential difference VBE1 across the base and emitter electrodes and the current I2 is applied to the thermal sensor 21 to cause a potential difference VBE2 across the base and emitter electrodes, a temperature calculation circuit 13 subtracts VBE2 from VBE1 and then generates a difference ΔVBE expressed in the following equation (1):
                              Δ          ⁢                                          ⁢                      V            BE                          =                                            V                              BE                ⁢                                                                  ⁢                1                                      -                          V                              BE                ⁢                                                                  ⁢                2                                              =                                                    KT                q                            ⁢                              ln                ⁡                                  (                                                            I                      1                                                              I                      2                                                        )                                                      +                                          (                                                      I                    1                                    -                                      I                    2                                                  )                            ⁢                              (                                                      R                    e                                    +                                                            R                      b                                        β                                                  )                                                                        (        1        )            
wherein K is Boltzmann's constant, T is the absolute temperature, q is the electron charge, Re is the series parasitic resistance of the base electrode, Rb is the series parasitic resistance of the emitter electrode, and β is the gain of the transistor. As a result, the potential difference ΔVBE generated by the temperature calculation circuit 13 is an analog signal that changes along with the temperature and therefore provides the information about the temperature. Afterwards, an analog-to-digital conversion circuit (ADC) 14 converts such analog signal into a digital temperature signal.
As seen in equation (1), the series parasitic resistances Re and Rb of the thermal sensor 21 causes a constant-term offset, (I1−I2)(Re+Rb/β), which is independent of the temperature. Hoping to get an accurate result on the temperature measurement, the prior art employs three or more different currents to sequentially excite the same thermal sensor 21 in order to eliminate the constant-term offset caused by such series parasitic resistances Re and Rb. However, the prior art three or more current excitation method not only requires a much higher frequency in operation but also causes some disadvantages like power inefficiency and temperature fluctuation. Even if the operational frequency is intentionally kept constant, the excitations by more and more currents will inevitably make each cycle of temperature measurement much longer and therefore reduce the speed of response, to the temperature variation, of the temperature measurement circuit 10.
On the other hand, what the temperature measurement circuit 10 actually monitors is the temperature of the semiconductor substrate on which the thermal sensor 21 is formed, and such actually monitored temperature may not necessary be equal to the real representative temperature of the external system 20. Especially in the case where the external system 20 is a computer, the temperature of interest would usually be the temperature of a thermal sinking plate 22 attached in the external system 20 instead of the temperature of the semiconductor substrate on which the thermal sensor 21 is formed. As for such case, the manufacturer of the external system 20 provides a temperature offset data ΔT, which indicates a temperature difference existing between the thermal sinking plate 22 and the substrate of the thermal sensor 21, to be stored in a register 15 of the temperature measurement circuit 10. Afterwards, the digital output of the analog-to-digital conversion circuit 14 are calibrated in accordance with the temperature offset data ΔT through an adder 16 so as to eventually generate an accurate temperature signal Tmp.